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Doctors classify blood pressure into four categories: normal, prehypertension (mild), stage 1 (moderate) and stage 2 (severe). Treatment depends on which category your pressure consistently falls in when readings are taken. The stages are based on the Joint National Committee 7 report done by the National Heart, Lung and Blood Institute, which is a part of the National Institutes of Health.

Blood pressure is measured with an instrument called a sphygmomanometer, through which the user listens for the sound of the force of blood in the patient’s arteries when the heart beats (systolic pressure). Measured in millimeters of mercury (mm Hg), systolic pressure is the top number in your blood pressure reading. The second, or bottom number, is the pressure in the arteries of the heart at rest — the diastolic pressure. Generally, as an adult, you are considered to have high blood pressure if your systolic pressure reading is greater than or equal to 140 mm Hg or if your diastolic pressure is greater than or equal to 90 mm Hg. But for every 20 mm Hg your systolic pressure raises above 115, and for every 10 mm Hg your diastolic pressure rises over 75, your risk of cardiovascular disease doubles — so lower pressures are generally better.

Stages of Hypertension

Here’s a look at the current stages and their recommended treatments:

Normal. Systolic less than 120 mm Hg and diastolic less than 80 mm Hg. No treatment is necessary, but you should monitor your blood pressure to be sure that it remains within the normal range.

Prehypertension: Systolic between 120 and 139 mm Hg or a diastolic between 80 and 89 mm Hg. “Prehypertension designates just what the term sounds like,” says Daniel Jones, MD, dean of the school of medicine at the University of Mississippi Medical Center in Jackson, Miss., and past president of the American Heart Association.. “It’s before someone crosses the threshold for the definition of hypertension but is at risk for developing hypertension. We don’t have evidence that using medications at this range is useful for preventing heart disease and stroke. However, because people in this group have some risk of moving on to developing heart disease, we recommend lifestyle measures to try to prevent the onset of hypertension.” Lifestyle measures include exercise, managing body weight into a normal range, eating a diet high in fruits and vegetables, and choosing low-fat dairy products.

Stage 1: Systolic between 140 and 159 mm Hg or diastolic 90 and 99 mm Hg. Management includes the same lifestyle measures as with prehypertension and the use of one of a number of drugs that are known to not only reduce blood pressure but also to reduce the risk of heart disease and stroke. Classes of drugs include: thiazide diuretics, ACE inhibitors, angiotensin receptor blockers, beta blockers, and calcium channel blockers. You may have to try different drugs until you find the one that has the best results for you. If you are African-American, you may be at higher than average risk for complications from hypertension. The JNC guidelines recommend that African Americans start with a regimen of two drugs if their top blood pressure reading is 145 mm Hg or higher.

Stage 2: Systolic 160 mm Hg or higher or diastolic 100 mm Hg or higher. In addition to lifestyle changes, “for many patients, it’s recommended that a two-drug therapy chosen from among the five classes of hypertensive agents be used to get their blood pressure down,” Dr. Jones says. “Again, you may have to experiment some to determine which combination of drugs works best for your body.

If your systolic and diastolic pressures fall into different stages, the stage with the higher number is the one that counts. For example, if you have a systolic pressure of 150 mm Hg but your diastolic pressure is only 85 mm Hg, you will be classified as stage 1 hypertension, not prehypertension. And if you are over age 50, it is the diastolic number that best predicts your risk of cardiovascular disease.

Left untreated, high blood pressure can lead to coronary heart disease, which can mean a heart attack or stroke. You should have your blood pressure checked regularly, and follow your doctor’s advice for keeping it under control.